The invention relates to a push rod, more specifically for use in a valve actuation device of an internal combustion engine, which is substantially hollow and is provided with a portion of a maximum cross section dimension arranged preferably in the middle between the ends of the push rod and to a method of manufacturing the same.
It has become known to make push rods for valve actuation devices of internal combustion engine hollow. As compared to solid push rods, they have the advantage of achieving better buckling resistance and strength on the one hand and lighter weight on the other hand.
U.S. Pat. No. 4,850,315 A and U.S. Pat. No. 5, 027,763 A disclose hollow push rods which have a portion of a maximum cross section dimension arranged between the two ends of the push rod. In the portion of a maximum cross section dimension, the outer diameter of the push rod is greater than in the end portions. This double conical shape achieves even better strength and buckling resistance. The push rod is thereby made in one piece and is fabricated from a tube in a cold forming process, the tube being conically formed by means of a tool on either side of the portion of a maximum cross section dimension. As described in U.S. Pat. No. 5,069,173 A, a hollow push rod may be fabricated in the same way, the hollow space thereof having an irregular cross section that departs from a circular line. The starting stock used thereby is a tube with an irregular inner cross section. This way of manufacturing a single piece push rod is quite complicated.
It is the object of the invention to provide a light weight push rod with high buckling resistance that can be easily manufactured.
This is achieved in accordance with the invention in that the push rod consists of a first and of a second substantially hollow rod piece, the rod pieces having a maximum cross section dimension in the region of one open front face thereof and being rigidly joined together at their open front faces.
Accordingly, the two rod pieces may be made separately, which makes it possible to design the outer shape and the hollow space in a flexible manner. The two rod pieces may hereby be joined by welding, preferably by electron-beam welding, or by gluing. Alternatively, the rod pieces may also be joined by pressing or screwing.
It is advantageous to build the two rod pieces according to the same technique for greatest ease of manufacture.
Particularly high buckling resistance may be achieved when the first rod piece and the second rod piece are substantially shaped like a cone or a cone segment, the respective one of the adjoining front faces of the rod pieces constituting the base of the cone. Portions that are susceptible to bending may thus be realized with a greater diameter. The highest surface pressures occur at the preferably closed ends of the push rod. In order to keep mechanical load on the one side and the weight on the other side as low as possible, it is provided that the wall thickness of the rod piece be greatest in the region of the preferably closed face opposite the open front face.
In order to permit load transfer to be as rigid as possible, for actuating the gas exchange valves of an internal combustion engine for example, there is provided that the push rod has, in axial direction, a spring stiffness of at least 30,000 N/mm in the direction of the push rod""s axis.
To further improve the strength of the push rod, there is provided, in a preferred embodiment, that the push rod be filled with a pressurized gas, with nitrogen preferably, the gas pressure within the push rod amounting to at least 50 bar, preferably to at least 100 bar.
After assembly, the push rod may thereby be filled with gas by way of a filling port that is specially provided for this purpose and subsequently closed. Alternatively, the two rod pieces may also be joined together in a pressurized workspace.